The present invention relates to a reflectometer, comprising an illuminator in a 45xc2x0/0xc2x0 configuration with a light source formed by a lamp and an illuminator optic, light emitted by the lamp being passed after reflection via a measuring opening into a housing and supplied to a measuring system contained therein, and the illuminator optic being formed by a number of light guides which start at the light source and end in the form of a rim with a conoidal emission side. More in particular, the invention relates to a reflectometer used in a construction as spectrophotometer, in which the measuring system can be conventionally formed by a spectrophotometer system with dispersion means, optical components cooperating therewith, and detection means.
Such a reflectometer is known from the international patent application WO 96/13709. For the illuminator in such a reflectometer designed as spectrophotometer, ISO standards have been set up; with respect to the measuring geometry for a reflection measurement the most recent ISO standards for color measurement, namely ISO-5-4:1995(E), give the 45xc2x0/0xc2x0 configuration, in which in the annular influx mode the illumination has to be effected on all sides at an angle of 45 (xc2x15)xc2x0 and the reflection measurement at 0 (xc2x15)xc2x0.
It is known to realize an illumination on all sides at an angle of 45xc2x0 by means of one or more lamps or LED""s which, provided or not provided with a lens, are directed from different directions to a specimen to be exposed. To comply with the most recent ISO standards, however, this requires a large number of lamps, which makes the construction of the spectrophotometer relatively expensive. It is also known to use one or more light sources in a diffusion chamber, the light radiating to the specimen at 45xc2x0 via an annular opening. The drawback of this solution is that such a diffusion chamber occupies relatively much space, which adversely affects the cost price and the dimensions of the spectrophotometer. Furthermore, U.S. Pat. No. 4,320,442 discloses an annular illuminator in which a system of reflectors is used, which are rather complicated owing to their shape. In the spectrophotometer as described in the opening paragraph, use is made of an illuminator in which the illuminator optic is formed by a number of optical guides which start at a light source and end in the form of a rim with a conoidal emission side. Such a spectrophotometer is known from U.S. Pat. No. 4,464,054. In the manufacture thereof, a rather labor-intensive finishing and mounting of a plurality of light guides in the form of glass fibers is necessary, both on the side of the light source and on the conoidal emission side.
The object of the invention is to provide the spectrophotometer with an illuminator optic which can be made in a simple and relatively inexpensive manner, and which illuminator optic also complies with the above-mentioned ISO standards.
According to the invention the reflectometer as described in the opening paragraph is characterized in that the illuminator optic consists of one group or of several groups of light guides, a group of light guides being made in one piece of plastic, said one piece having an entrance area, a transition area for gradually merging the entrance area into a number of mechanically separated light guides and an annular emission area in which the light guides are mechanically connected with each other. This solution enables inexpensive mass production of the illuminator optic, in particular by using injection molding techniques. It may be noticed that from U.S. Pat. No. 4,518,259 an illuminator optic, made from plastic, is known per se.
The illuminator optic is based on the principle of total internal reflection in the light guides; as a result of the difference ill refractive index with the surrounding air. This ensures that light radiated in can be transported practically without losses, as long as the geometric conditions for total reflection remain satisfied. In connection therewith, it is important that the illuminator optic contacts the further housing of the spectrophotometer in as few places as possible.
In connection with the geometric conditions imposed on the light guides, the composition of the material of the light guides, their thickness and curvature are adjusted to the angles at which light can enter the glass guides in such a manner that the requirements for total internal reflection are nearly completely satisfied.
For each group of light guides a lamp could be present. From considerations of cost, however, it is preferred to make use of only one lamp. Similarly, it is preferred if the illuminator optic is formed by only one group of light guides. The illuminator optic may then be formed by, for instance, one single injection molded part.
Preferably, the lamp is rigidly connected with the illuminator optic. Not only does this result in a saving of mounting cost, but an accurately positioned fastening method for the lamp with respect to the illuminator optic is obtained. The lamp can be directly attached to the illuminator with optically bright cement, thereby excluding both light losses through reflections and through possible soiling and alignment errors, while, furthermore, the stability of the light intensity radiated in becomes insensitive to unintentional displacement of the lamp. When using a separate lamp foot or supporting construction, displacements can be caused by, for instance, vibrations and warming up.
In a preferred embodiment the lamp is formed by a Xenon flash tube. Such tubes have advantages over the light bulbs frequently used in known spectrophotometers. The emitted light properly corresponds with daylight, while light bulbs contrarily have very little blue light and even less UV-radiation. In particular if the detection means are provided with silicon photodiodes, which actually have a low sensitivity to short wavelengths, in particular blue light and UV-radiation, Xenon flash tubes form an ideal light source. The cost price is low and the light output is high. If the flash power is properly selected, the life can be tens of millions of pulses. The diffuse light from a Xenon tube, however, cannot be simply directed to a specimen, while through their dimensions and shape and the required high-voltage feed they cannot be simply arranged close to a specimen either. To comply with the above-mentioned xe2x80x9cannular 45xc2x0/0xc2x0 ISO standardxe2x80x9d, diffusion chambers, various reflectors and so-called integrating spheres were used in the past. All these solutions, however, are relatively expensive and occupy much space, with the result that a Xenon illumination is particularly used in larger and more expensive apparatus. The illuminator optic according to the invention, however, offers a solution for the use of Xenon flashlight which occupies little space and, moreover, can be made and arranged at low cost. The possibilities of the illuminator optic, however, are not limited to the combination with a Xenon tube. Also when using it with other sources, tubular or not tubular, a high illumination output can be obtained for an xe2x80x9cannular 45xc2x0/0xc2x0xe2x80x9d illumination with a form of the entrance area adapted to the light source, while only one light source is required or one LED of each color to be used.
The lamp is preferably tubular and completely surrounded by a reflector with the exception of a slit on the side of the entrance area of the illuminator optic. The Xenon tube may, as already mentioned, be glued to the entrance side of the illuminator optic. By further surrounding the lamp by a reflector, it is achieved that as little light as possible is emitted in directions other than to the illuminator optic.
In a specific design the light entrance of the entrance area of the illuminator optic has a rectangular cross-section with a thickness greater than that of the channels. The transition area following the light entrance provides a conical course, the channels, with decreasing thickness of the entrance area, being formed from the rectangular cross-section while diverging widthwise. This design of the transition area, in which the thickness of the entrance area decreases according as the width of the beam of rays increases, efficiently couples the shape of the light source to these light channels, which first diverge groupwise and then support each other in other combinations in some places. These contact areas serve to give the construction sufficient strength, so that the illuminator optic can be manufactured reliably and at low cost. The contact areas have been selected in combination with the angles of flexure of the channels in such a manner with respect to the effective optical width of the channels that the light is gradually deflected everywhere and the conditions of total reflection are satisfied as much as possible. This prevents too much light exit and maximizes the efficiency.
It is necessary for the firmness of the illuminator optic to interconnect the channels at the exit. In the emission area this causes a light scattering from the end of the channels. It has been found that the diffuse light can be concentrated somewhat on the site in the direction of the specimen by giving the ends of the channels a convex lens-shaped design.
The light emitted via the exits of the channels is conoidally directed at an angle of 45xc2x0, and the thickness of the lens-shaped exit openings of the channels, measured perpendicularly to the light direction, is less than 17.5% of the distance from the exit of the channels to the center of the exposed surface.
The illuminator optic is made of an optically bright plastic, preferably acrylate glass (PMMA).
It has been found that the intensity of the scattered light from the illuminator optic is representative of the intensity of the reflected light. To enable allowance for variations therein and for the effect of these variations on the final measuring results, detection means are arranged near the illuminator optic to measure the scattered light from the illuminator optic and, in dependence thereon, to make corrections to the measuring results given by the measuring system. The detection means are formed by a diffuser and a photodetector and, because the scattered light is most intense where the channels are most curved, are arranged near the most curved parts of the illuminator optic.
The reflectometer according to the invention is an inexpensive, handy and reliable instrument for measuring surface colors of printing matter, paint, plastics, textiles, foodstuffs, etc. The measuring system, in particular the spectrophotometer system, can be connected to a computer so as to be able to process the information from this measuring system.